1. Field of the Invention
The present invention relates generally to high speed electrical communication links, and more particularly to reducing cross talk between electrical channels in serial link buses in personal computers, servers, switches and routers.
2. Description of the Related Art
A typical data communication link used in, for example, a computer includes one or more electrical data buses which may connect a first chip with a second chip. The electrical bus may be routed over solder balls, plated through holes normally referred to as vias, and conductive paths configured as transmission lines such as microstrips or strip lines, all of which affect the electrical performance of the communication link.
An important element in the performance of the communication link is the amount of cross talk which is present. Cross talk is the undesired coupling of signal energy from one conducting path (aggressor path) to another conducting path (victim path). This coupling is due to the physical proximity of the various conducting parts, specifically the microstrips or strip lines, the vias and/or wires inside of connectors. More particularly, cross talk between a circuit on a printed circuit board on one of the lines in the bus and a circuit on a down stream printed circuit board on another line in the bus is detrimental to the quality of the received signals. Usually this cross talk increases with frequency and thus limits the maximum achievable bit rate on the bus.
Point to point serial link busses are becoming the way to connect VLSI chips, e.g., microprocessors, memory buffers, bridge chips, switches, etc., amongst each other and to the rest of a system. Examples are FDB1 and FBD2, Intel QuickPath, AMD Hyper Transport, IBM Eleastic Interface etc. These interconnect solutions are characterized by combining individual serial links to a serial link bus and adding a forwarded clock signal to this bus.
As the data rate of these links reach multi-Gbps data rates, the intersymbol interference caused by dispersion (band limited) electrical channel and the timing jitter are no longer the only source of eye closure. In addition to these issues, the cross talk between electrical channels becomes a major issue, in particular in the case of single ended signaling.
Known solutions to cross talk suppression or equalization are Multiple Input, Multiple Output (MIMO) links. These links are very complicated to implement and do not fit the power/area envelop of serial link busses. Another solution averages Cross decision Feedback Equalization (CFE) where the contributions of the cross talk terms are determined from detected bits in the aggressor lanes (the cross talk source) and the derived voltage is subtracted from the input of the victim lane (the cross talk recipient). This solution has severe timing limitations. Another solution is based on predistortion in the transmitter, where the cross talk terms that are incident to the receiver are already subtracted from the victim lane in the transmitter. However, this significantly reduces the signal swing of the victim lane due to limited maximum signal swing provide by the available power supply. Other measures used to reduce cross talk include differential signaling which involves the use of two adjacent interconnects to transmit one signal, ground shields, increasing the physical separation between adjacent conductors, passive and active cross talk compensation and the like. Cross talk compensation general refers to any structure, device, or circuit which introduces energy coupling between interconnects of opposite polarity to partially cancel out the undesired cross talk. Cross cancellation is normally done in close proximity to the location where the cross talk occurs such as in a connector or on a chip, or it may be at a location along the communication link.